BACKGROUND: Epidermal growth factor (EGF) is the founding member of the EGF family of mitogens possessing potent biological activities on numerous cell types. Other notable members of the family include HB-EGF, TGF-alpha, and four neuregulin isoforms. Human EGF is synthesized as a large precursor (1,208 amino acids), which is cleaved to generate the mature form containing 53 amino acids with three intramolecular disulfide linkages mediated by six highly conserved cysteine residues. The mature EGF peptide binds with high affinity to EGF receptor (ErbB) and elicits a signaling cascade, ultimately promoting DNA replication and cell proliferation. While insufficient ErbB signaling can cause degenerative diseases, excessive receptor activation is associated with oncogenic development. The recombinant human EGF is expressed as a GST-fusion protein. The fusion junction of the protein harbors an engineered thrombin cleavage site and a protein kinase A (PKA) phosphorylation site. The GST portion of the fusion protein can be cleaved off by thrombin if necessary. The fusion protein can be radioactively labeled with 32P-r-ATP and PKA.